Multi-piece solid golf ball

ABSTRACT

A multi-piece solid golf ball has a core, an intermediate layer encasing the core, and a cover layer which encases the intermediate layer and has a plurality of dimples on an outer surface thereof. The intermediate layer is formed primarily of a material which includes specific amounts of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500 and a basic inorganic metal compound capable of neutralizing un-neutralized acid groups per 100 parts by weight of a resin component composed primarily of a specific olefin-unsaturated carboxylic acid copolymer, and in which at least 82 mol % of acid groups in the above components are neutralized. The cover layer is formed of a thermoplastic resin composed primarily of an ionomer resin, and has a Shore D hardness of from 50 to 85. This invention makes it possible to achieve both an increased distance owing to a reduced spin rate, and also an improved durability.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball of three or more pieces which has a core, an intermediate layer and a cover. More specifically, the invention relates to a multi-piece solid golf ball which achieves an increased distance owing to a reduced spin rate, and also has an improved durability.

Low-compression golf balls are known to have a poor durability. As a result, the use of ionomers, which are resistant to cracking and relatively soft, as the cover material has become commonplace.

On the other hand, in order to further increase the distance traveled by the ball, it is necessary to make the cover harder and thereby lower the spin rate of the ball. In such cases, when a hard ionomer is used as the cover, the cover has a tendency to crack, in addition to which the ball durability becomes very poor. Also, if the spin rate of the ball is reduced too much, the lift of the ball after being struck will decrease, possibly resulting in a poor carry.

In this connection, JP-A 2000-61001 discloses art relating to a golf ball having an excellent flight performance and durability, and also endowed with a very soft and good feel on impact, which ball has a four-piece construction composed of a core, an envelope layer, an intermediate layer and a cover, each of the respective pieces being made of specific materials and having specific hardnesses and gauges. In addition, JP-A 2001-218872, in order to provide a golf ball having an excellent feel, controllability, durability and flight performance, discloses a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, wherein the envelope layer is formed primarily of, e.g., a polyester thermoplastic elastomer and an inert filler has been added to the cover.

However, the foregoing conventional golf balls leave something to be desired in terms of improving the increase in distance while maintaining a good durability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which achieves an increased distance owing to a reduced spin rate, and also has an improved durability.

The inventors have conducted extensive investigations in order to attain the above object. As a result, they have discovered that, in a multi-piece solid golf ball having a core, an intermediate layer and a cover, by using a hard ionomer material as the cover material in order to lower the spin rate and by also selecting a highly neutralized ionomer resin composition as the intermediate layer material, the adherence between the intermediate layer and the cover increases, resulting in a dramatic improvement in the durability to cracking. Moreover, a lower spin rate is achieved on full shots, increasing the distance traveled by the ball. This discovery ultimately led to the present invention.

In this invention, because the cover has a very strong tendency to crack, particularly in those cases where a high-acid ionomer is used as the cover material, the durability can be increased by adding a granular inorganic filler.

One of the drawbacks of using a hard cover material is that the feel on impact worsens. In addition, a hard cover causes the ball to separate rapidly from the club when struck with a W#1, leaving the golfer with an impression that the ball is difficult to control. Therefore, to improve the feel of the ball on impact, it is preferable to provide between the intermediate layer and the core a relatively soft elastomer layer, i.e., an envelope layer. As a result, a soft feel and good controllability can be obtained.

In addition, when the spin rate of the golf ball is reduced, the ball has less lift and thus tends to have a lower trajectory; a lower trajectory has the undesirable effect of shortening the carry. Hence, in this invention, it is desirable to use dimples having a relatively large lift. Specifically, it is preferable to use a relatively small number of dimples; i.e., a number of dimples within a range of from 250 to 340.

Accordingly, the invention provides the following multi-piece solid golf ball.

[1] A multi-piece solid golf ball comprising a core, an intermediate layer encasing the core, and a cover layer which encases the intermediate layer and has a plurality of dimples on an outer surface thereof, wherein the intermediate layer is formed primarily of a material which is comprised of:

100 parts by weight of a resin component composed of, in admixture,

-   -   a base resin of (a) an olefin-unsaturated carboxylic acid random         copolymer and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer blended         with (b) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer in a         weight ratio between 100:0 and 0:100, and     -   (e) a non-ionomeric thermoplastic elastomer in a weight ratio         between 100:0 and 50:50;

(c) from 5 to 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and

(d) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c),

and in which at least 82 mol % of acid groups in the base resin and component (c) are neutralized; and wherein the cover layer is formed of a thermoplastic resin composed primarily of an ionomer resin and has a Shore D hardness of from 50 to 85. [2] The multi-piece solid golf ball of [1], wherein the ionomer resin in the cover layer includes at least 16 wt % of an unsaturated carboxylic acid, and the cover layer includes from 5 to 35 parts by weight of a granular inorganic filler per 100 parts by weight of the thermoplastic resin. [3] The multi-piece solid golf ball of [1], which further comprises, between the core and the intermediate layer, an envelope layer composed primarily of a polyester elastomer. [4] The multi-piece solid golf ball of [1], wherein the total number of dimples is from 250 to 342.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golf ball according to an embodiment of the invention.

FIG. 2 is a plan view of Dimple Configuration I on the ball surface used in the examples of the invention and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below in conjunction with the diagrams. The multi-piece golf ball G of the invention, as shown in FIG. 1, is formed as a structure of at least three pieces which includes a core 1, an intermediate layer 3 encasing the core, and a cover 4 encasing the intermediate layer 3. Although not shown in this diagram, a large number of dimples are formed on the surface of the cover 4. In addition, in FIG. 1, an envelope layer 2 is formed between the core 1 and the intermediate layer 3. Here, FIG. 1 shows a structure in which the core 1, the intermediate layer 2 and the cover 3 are each composed of a single layer, although any of these may be composed of a plurality of two or more layers. In cases where the core, intermediate layer and/or cover described below are respectively composed of a plurality of layers, the plurality of layers making up the core, the intermediate layer and/or the cover should be constituted in such a way as to satisfy, as a whole, the requirements for that portion of the ball.

The rubber material is exemplified by a rubber composition which contains a base rubber and additionally includes, for example, a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. Preferably, polybutadiene is used as the base rubber of this rubber material.

It is desirable for the polybutadiene to have a cis-1,4 bond content on the polymer chain of at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4 bond content among the bonds on the molecule may result in a lower resilience.

Also, the polybutadiene has a 1,2-vinyl bond content on the polymer chain of generally not more than 2%, preferably not more than 1.7%, and more preferably not more than 1.5%. Too high a 1,2-vinyl bond content may result in a lower resilience.

To obtain a molded and vulcanized rubber composition which has a good resilience, the polybutadiene used in this invention is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

In this invention, in particular, the use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Preferred examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

To increase the resilience, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.

Rubber components other than the above polybutadiene may be included in the base rubber insofar as the objects of the invention are attainable. Illustrative examples of rubber components other than the above polybutadiene include other polybutadienes, and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with desired metal ions. Specific examples include the zinc salts and the magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The amount of unsaturated carboxylic acid and/or metal salt thereof included per 100 parts by weight of the base rubber is set to preferably at least 2 parts by weight, more preferably at least 4 parts by weight, and even more preferably at least 6 parts by weight. The upper limit is set to preferably not more than 60 parts by weight, more preferably not more than 45 parts by weight, even more preferably not more than 35 parts by weight, and most preferably not more than 25 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.

The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (available from NOF Corporation), Perhexa C-40 and Perhexa 3M (both available from NOF Corporation), and Luperco 231XL (Atochem Co.). These may be used singly or as a combination of two or more thereof.

The amount of organic peroxide included per 100 parts by weight of the base rubber may be set to preferably at least 0.1 part by weight, more preferably at least 0.3 part by weight, even more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight. The upper limit may be set to preferably not more than 5 parts by weight, more preferably not more than 4 parts by weight, even more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel, durability and rebound.

Examples of preferred inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly or as a combination of two or more thereof.

The amount of inert filler included per 100 parts by weight of the base rubber may be set to preferably at least 1 part by weight, and more preferably at least 5 parts by weight. The upper limit may be set to preferably not more than 200 parts by weight, more preferably not more than 150 parts by weight, and even more preferably not more than 110 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a suitable rebound.

In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6, Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly or as a combination of two or more thereof.

The amount of antioxidant included may be more than 0, and is set to preferably at least 0.05 part by weight, and especially at least 0.1 part by weight, per 100 parts by weight of the base rubber. The upper limit, although not subject to any particular limitation, may be set to preferably not more than 3 parts by weight, more preferably not more than 2 parts by weight, even more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight, per 100 parts by weight of the base rubber. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.

To enhance the rebound of the golf ball and thereby increase its initial velocity, an organosulfur compound may be included in the above base rubber. No particular limitation is imposed on the organosulfur compound, provided it improves the rebound of the golf ball. Exemplary organosulfur compounds include thiophenols, thionaphthols, halogenated thiophenols, and metal salts thereof. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zinc salt of pentabromothiophenol, the zinc salt of p-chlorothiophenol; and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The zinc salt of pentachlorothiophenol is especially preferred.

In order to obtain the subsequently described desired properties, the above core (hot-molded material) may be fabricated by suitably selecting the above-described rubber composition and subjecting it to vulcanization and curing by a method similar to that used with known golf ball rubber compositions. Vulcanization may be carried out under, for example, the following conditions: vulcanization temperature, 100 to 200° C.; vulcanization time, 10 to 40 minutes.

The diameter of the solid core in the invention is not subject to any particular limitation, although it is recommended that it be preferably at least 25 mm, more preferably at least 28 mm, and even more preferably at least 30 mm, and that the upper limit be preferably not more than 40 mm, more preferably not more than 39 mm, and even more preferably not more than 38 mm. If the core diameter is small, the feel on impact may become hard. If the diameter is large, the intermediate layer and cover will inevitably be thinner, which may worsen the durability.

The above core has a deflection, when compressed under a final load of 130 kgf from an initial load of 10 kgf, of preferably at least 2.5 mm, more preferably at least 3.0 mm, and even more preferably at least 3.5 mm. The upper limit is preferably not more than 6.0 mm, more preferably not more than 5.5 mm, and even more preferably not more than 5.0 mm. If the deflection of the solid core is too small, the feel on impact may worsen and, particularly on long shots such as with a driver in which the ball incurs large deformation, may subject the ball to an excessive rise in the spin rate, shortening the distance traveled by the ball. On the other hand, if the ball is too soft, the ball may have a dead feel and a less than adequate rebound, shortening the distance traveled, in addition to which the ball may have a poor durability to cracking on repeated impact.

Next, in the present invention, an ionomer composition in which components (a) and (b) serve as the base resin is used as the intermediate layer material.

The base resin is composed of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer, and (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer, which components (a) and (b) are mixed in a weight ratio between 100:0 and 0:100.

Components (a) and (b) are olefin-containing copolymers. The olefin(s) in these components has a number of carbons which may be set to at least 2 but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of the unsaturated carboxylic acid(s) in component (a) or component (b) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester in component (b) is preferably a lower alkyl ester of the above unsaturated carboxylic acid. Specific examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

The random copolymer in component (a) or component (b) of the invention can be obtained by random copolymerization of the above components using a known method. Here, it is recommended that the content of unsaturated carboxylic acid (acid content) included in the random copolymer be generally at least 2 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %, and that the upper limit be not more than 25 wt %, preferably not more than 20 wt %, and even more preferably not more than 15 wt %. If the acid content is low, the rebound may decrease, and if it is high, the processability of the material may decrease.

The metal salt of the copolymer in component (a) or component (b) may be obtained by neutralizing some of the acid groups in the random copolymer of component (a) or component (b) with metal ions.

Here, examples of metal ions which neutralize acid groups include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Of these, preferred use may be made of Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺. The use of Zn⁺⁺ is especially preferred. The degree of neutralization of the random copolymer by these metal ions, although not subject to any particular limitation, is generally at least 82 mol %, and preferably at least 85 mol %. The upper limit is not more than 120 mol %, and preferably not more than 110 mol %. At a degree of neutralization in excess of 110 mol %, the moldability may decrease. On the other hand, at less than 82 mol %, it becomes necessary to increase the amount in which the inorganic metal compound serving as component (d) is added, which may be disadvantageous in terms of cost, and also makes it impossible to achieve a sufficient rebound. Such a neutralization product may be obtained by a known method. For example, it may be obtained by introducing a compound such as a formate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide or alkoxide of any of the above-mentioned metal ions into the above-described random copolymer.

Illustrative examples of the olefin-unsaturated carboxylic acid random copolymer making up component (a) include the products available from DuPont-Mitsui Polychemicals Co., Ltd. under the trade names Nucrel 1560, Nucrel 1525 and Nucrel 1035. Illustrative examples of metal salts of the olefin-unsaturated carboxylic acid random copolymers include the products available from DuPont-Mitsui Polychemicals Co., Ltd. under the trade names Himilan 1605, Himilan 1601, Himilan 1557, Himilan 1705 and Himilan 1706, and the products available from E.I. DuPont de Nemours & Co. under the trade names Surlyn 7930 and Surlyn 7920.

Illustrative examples of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer making up component (b) include the products available from DuPont-Mitsui Polychemicals Co., Ltd. under the trade names Nucrel AN4318, Nucrel AN4319 and Nucrel AN4311. Illustrative examples of metal salts of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer include the products available from DuPont-Mitsui Polychemicals Co., Ltd. under the trade names Himilan AM7316, Himilan AM7331, Himilan 1855 and Himilan 1856, and the products available from E.I. DuPont de Nemours & Co. under the trade names Surlyn 6320 and Surlyn 8120.

The mixing ratio between above components (a) and (b), expressed as a weight ratio, is preferably between 100:0 and 0:100, more preferably between 0:100 and 40:60, and even more preferably between 0:100 and 20:80.

In addition to the above essential ingredients, (e) a non-ionomeric thermoplastic elastomer may be added in order to improve the feel of the ball when struck. Admixture may be carried out so that the weight ratio of the above base resin to the non-ionomeric thermoplastic elastomer (e) is between 100:0 and 50:50.

Various types of non-ionomeric thermoplastic elastomers may be included as the non-ionomeric thermoplastic elastomer (e). Examples of such non-ionomeric thermoplastic elastomers include styrene thermoplastic elastomers, ester thermoplastic elastomers and urethane thermoplastic elastomers. The use of a styrene thermoplastic elastomer is especially preferred.

In addition, the intermediate layer material also includes, per 100 parts by weight of the above-described resin component:

(c) from 5 to 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500, and (d) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the above base resin and component (c).

Above component (c) is a fatty acid or a fatty acid derivative having a molecular weight of at least 280 but not more than 1500. This component helps to improve the flowability of the hot mixture; because it has a very low molecular weight compared with the above resin component, it helps to markedly increase the melt viscosity of the mixture. Moreover, because the fatty acid (or fatty acid derivative) in component (c) has a molecular weight of at least 280 but not more than 1500 and includes a high content of acid groups (or derivatives thereof), any loss of resilience due to its addition will be small.

The fatty acid or fatty acid derivative of component (c) which is used in this invention may be an unsaturated fatty acid (or derivative thereof) containing a double bond or triple bond on the alkyl moiety, or a saturated fatty acid (or derivative thereof) in which the bonds on the alkyl moiety are all single bonds. It is recommended that the number of carbons on the molecule be typically at least 18, and that the upper limit be not more than 80, and especially not more than 40. A smaller number of carbons may result in a poor heat resistance and may also result in an acid group content so high as to make it impossible to obtain the desired flowability on account of interactions with acid groups present in the base resin. On the other hand, a larger number of carbons increases the molecular weight, which may lower the flowability, possibly making use as a material difficult.

The fatty acid or fatty acid derivative of component (c), although not subject to any particular limitation, is preferably of one or more type selected from the group consisting of stearic acid, behenic acid, oleic acid, maleic acid, and metal salts thereof.

The fatty acid derivative of component (c) is exemplified by fatty acid derivatives in which the proton on the acid group of the fatty acid has been substituted. Illustrative examples include metal soaps in which the proton has been replaced with a metal ion. The metal salt may be one which uses a metal ion having a valence of 1 to 3, the metal ion being preferably selected from the group consisting of lithium, sodium, magnesium, aluminum, potassium, calcium and zinc. The use of a metal salt of stearic acid is especially preferred. Specifically, the use of magnesium stearate, calcium stearate, zinc stearate or sodium stearate is preferred. Of these, the use of magnesium stearate is especially preferred.

Component (c) of the present invention is included in an amount, per 100 parts by weight of the base resin, of at least 5 parts by weight, preferably at least 15 parts by weight, more preferably at least 40 parts by weight, and even more preferably at least 60 parts by weight. The upper limit is not more than 150 parts by weight, preferably not more than 130 parts by weight, more preferably not more than 120 parts by weight, and even more preferably not more than 100 parts by weight.

When using above-described components (a) and (b), use may also be made of known metallic soap-modified ionomers (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and WO 98/46671) when using above-described components (a) and/or (b), and component (c).

Component (d) is a basic inorganic metal compound capable of neutralizing acid groups in the base resin and in component (c). When the above base resin and component (c) alone, and in particular a metal-modified ionomer resin alone (e.g., a metal soap-modified ionomer resin cited in the above-mentioned patent publications, alone), is mixed under heating, as shown below, the metallic soap and un-neutralized acid groups present in the ionomer undergo exchange reactions, generating a fatty acid. Because the fatty acid thus generated has a low thermal stability and readily vaporizes during molding, not only does it cause molding defects, in cases where the fatty acid that has formed deposits on the surface of the molded product, it may substantially lower paint film adherence. Component (d) is included in order to resolve such problems.

As described above, the hot mixture used in the invention includes, as an essential component, (d) a basic inorganic metal compound which neutralizes acid groups in the base resin and component (c). By including component (d), acid groups in the base resin and component (c) are neutralized. Moreover, synergistic effects from the blending of these respective components increases the thermal stability of the hot mixture and at the same time confers a good moldability, contributing to the rebound of the golf ball.

Component (d) is a basic inorganic metal compound which is capable of neutralizing acid groups in the base resin and component (c). It is recommended that component (d) be preferably a monoxide or a hydroxide. Because it has a high reactivity with ionomer resins and the reaction by-products contain no organic compounds, this component is able to increase the degree of neutralization of the hot mixture without a loss of thermal stability.

Illustrative examples of the metal ion used here in the basic inorganic metal compound include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Basic inorganic fillers containing these metal ions may be used as the basic inorganic metal compound. Specific examples include magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. As noted above, a hydroxide or a monoxide is preferred. The use of magnesium oxide or calcium hydroxide, which have a high reactivity with the ionomer resin, is especially preferred.

The amount of component (d) included per 100 parts by weight of the base resin is from 0.1 to 17 parts by weight. This amount is preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 2 parts by weight. The upper limit is preferably not more than 17 parts by weight, more preferably not more than 15 parts by weight, and even more preferably not more than 13 parts by eight.

It is recommended that, in the hot mixture used in the invention, preferably at least 82 mol %, more preferably at least 85 mol %, and even more preferably at least 90 mol %, but not more than 120 mol %, and preferably not more than 110 mol %, of the acid groups in the base resin and component (c) be neutralized. By means of high neutralization, the exchange reactions which become a problem when the base resin and fatty acid (or fatty acid derivative) described above alone are used, are more reliably suppressed, making it possible to prevent the generation of fatty acid and thus markedly increasing the thermal stability, enabling a material having a good moldability and a greatly increased resilience compared with prior-art ionomer resins to be obtained.

Here, with regard to neutralization of the hot mixture, of the invention, to more reliably achieve both a high degree of neutralization and flowability, it is recommended that the acid groups in the hot mixture be neutralized by transition metal ions and alkali metal and/or alkaline earth metal ions. Because transition metal ions have a weaker ionic cohesion than alkali metal ions and alkaline earth metal ions, they neutralize some of the acid groups in the hot mixture, enabling a substantial improvement in flowability to be achieved.

In addition, various additives, such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers, may be optionally added to the hot mixture.

The method of preparing the hot mixture is exemplified by mixture under heating at a temperature of between 150 and 250° C. in an internal mixer such as a twin-screw extruder, a Banbury mixer or a kneader. The method of forming the intermediate layer using the hot mixture is not subject to any particular limitation. For example, the intermediate layer may be formed by injection molding or compression molding the hot mixture. When injection molding is employed, the process used may be one that involves placing a prefabricated core at a given position in the injection mold, then introducing the above material into the mold. When compression molding is employed, the process may involve producing a pair of half cups from the above material, covering the core with these half-cups, either directly or over an intervening intermediate layer, then applying pressure and heat within a mold. If molding under heat and pressure is carried out, the molding conditions may be a temperature of from 120 to 170° C. and a period of from 1 to 5 minutes.

The material hardness of the intermediate layer of the invention, although not subject to any particular limitation, is preferably at least 30, more preferably at least 35, and even more preferably at least 40, but preferably not more than 60, more preferably not more than 58, and even more preferably not more than 55. If the Shore D hardness is lower than the above range, the resilience may decrease, possibly resulting in a shorter distance. On the other hand, if the Shore D hardness is higher than the above range, the durability to cracking may be poor.

The gauge of the intermediate layer is not subject to any particular limitation, although it is recommended that the intermediate layer be formed to a gauge of preferably at least 0.5 mm, more preferably at least 0.8 mm, and even more preferably at least 1.0 mm, but preferably not more than 3.0 mm, more preferably not more than 2.5 mm, and even more preferably not more than 2.0 mm. If the intermediate layer is too thick, it may be impossible to improve the feel of the ball and the flight performance. On the other hand, if it is too thin, the flight performance and durability may worsen.

In this invention, as shown in FIG. 1, it is preferable to form an envelope layer 2 between the core 1 and the intermediate layer 3. In this case, the envelope layer material is not subject to any particular limitation, although preferred use may be made of various types of thermoplastic elastomers, or of a mixture of a thermoplastic elastomer and an ionomer resin. Exemplary thermoplastic elastomers include various types of elastomers, such as polyester-based, polyamide-based, polyurethane-based, polyolefin-based and polystyrene-based elastomers. From the standpoint of conferring flexibility and excellent resilience, the use of a polyester-based thermoplastic elastomer is especially preferred.

It is preferable for the above thermoplastic elastomer to be included within the envelope layer material in an amount of generally at least 50 wt %, and especially at least 70 wt %. In this case, it is desirable to not include other resins in the envelope layer material. If the amount of the above thermoplastic elastomer included is low, the ball may have an inadequate rebound and the distance may decrease.

In cases where an envelope layer is provided, the envelope layer has a material hardness, expressed as the Shore D hardness, which, although not subject to any particular limitation, is typically at least 30, and preferably at least 35, but typically not more than 50, and preferably not more than 45.

Next, the cover layer serving as the outermost layer of the invention is described. The cover layer material is not subject to any particular limitation, although the use of a thermoplastic resin composed primary of an ionomer resin is preferred. In this case, the ionomer resin used may be an ethylene-α,β-unsaturated carboxylic acid copolymer which includes at least 16 wt %, and preferably from 18 to 25 wt %, of an α,β-unsaturated carboxylic acid and is neutralized with metal ions, particularly metal ions selected from among lithium, sodium, potassium, magnesium, zinc, copper, barium, lead and aluminum. In cases where the α,β-unsaturated carboxylic acid content is less than 16 wt %, a sufficient hardness may not be attainable. Preferred use may be made of an unsaturated carboxylic acid having from 2 to 8 carbons; acrylic acid and methacrylic acid are especially preferred. The degree of neutralization by the above metal ions is preferably from 10 to 100 mol %, and especially from 20 to 80 mol %. Blending and using together an ionomer having such a high acid content with a low acid content ionomer containing not more than 15 wt % of an α,β-unsaturated carboxylic acid is desirable for ensuring the durability to cracking.

A given amount of a granular inorganic filler may be included in the cover layer. For example, zinc oxide, calcium carbonate, barium sulfate, titanium oxide and the like may be used as the granular inorganic filler. In particular, by using these in combination, it is possible to prevent discoloration of the cover layer and to enhance durability.

In particular, in this invention, from the standpoint of enhancing durability, it is preferable to use precipitated barium sulfate as the chief ingredient of the granular inorganic filler. In this case, the amount of precipitated barium sulfate included is preferably at least 60 wt %, more preferably at least 70 wt %, and even more preferably at least 80 wt %, of the total amount of granular inorganic filler included. The upper limit in the amount of precipitated barium sulfate included is up to 100 wt %, and preferably not more than 95 wt %, of the total amount of granular inorganic filler included. By including a small amount of granular inorganic filler other than precipitated barium sulfate, the discoloration resistance, resilience and durability of the cover can be further enhanced.

The amount of the above granular inorganic filler included is preferably in a range of from 5 to 35 parts by weight per 100 parts by weight of the thermoplastic resin used in the cover material. The lower limit is more preferably at least 10 parts by weight, and the upper limit is more preferably not more than 30 parts by weight, and even more preferably not more than 25 parts by weight. In excess of this upper limit in the amount included, the durability to cracking may decrease or the distance traveled by the ball may decrease.

The Shore D hardness of the cover layer, although not subject to any particular limitation, is preferably at least 50, more preferably at least 55, and even more preferably at least 69. The upper limit is preferably not more than 85, and more preferably not more than 80. If the Shore D hardness of this cover layer falls outside of the above range, the spin rate may increase, as a result of which the desired distance may not be attainable, in addition to which the durability to cracking may be diminished.

The gauge of the cover layer, although not subject to any particular limitation, is preferably at least 0.5 mm, and more preferably at least 0.7 mm. The upper limit is preferably not more than 2.0 mm, and more preferably not more than 1.4 mm. If the cover layer gauge falls outside of the above range and the layer is too thin, the durability to cracking may worsen. On the other hand, if the cover layer is too thick, the spin rate of the ball when struck may become too high, lowering the distance traveled by the ball.

In the golf ball of the invention, to further improve the aerodynamic properties and thereby increase the distance, as in conventional golf balls, it is desirable to form a plurality of dimples on the surface of the ball. By optimizing dimple parameters, such as the types and total number of dimples, synergistic effects with the above-described ball construction render the trajectory more stable, making it possible to obtain a golf ball having an excellent distance performance. Moreover, the ball surface may be subjected to various types of treatment, such as surface preparation, stamping and painting, in order to enhance the design and durability of the golf ball.

With regard to the number of dimples formed on the ball surface, in order to lower the spin rate and lower the trajectory, it is preferable to have a relatively small number of dimples with a large lift. Specifically, the number of dimples in the present invention is set to preferably at least 250, and the upper limit is set to preferably not more than 342, and more preferably not more than 332.

The shapes of the dimples are not limited to circular shapes; one or more type from among, for example, various polygonal shapes, dewdrop shapes and oval shapes may be suitably selected. Employing dimple shapes such that the distance between neighboring dimples (land width) becomes substantially 0 is desirable in that the surface coverage can be increased.

To fully manifest the aerodynamic characteristics of the dimples, the dimple coverage on the spherical surface of the golf ball, which is the sum of the individual dimple surface areas, each defined by the margin of the flat plane circumscribed by the edge of a dimple, expressed as a ratio (SR) with respect to the spherical surface area of the ball were it to be free of dimples, is preferably at least 75%.

Generally, by intermingling large and small dimples, it is possible to increase the surface coverage. Also, it is desirable to combine dimples having contour lengths of from 7 to 20 mm. Dimples having the same shape but differing depths may also be mixed. By setting the number of such dimples types to five or more, symmetry can be reliably achieved.

To enhance the aerodynamic performance and achieve the desired objects of the invention, the total volume of the dimples, although not subject to any particular limitation, may be set to preferably at least 400 mm³, and more preferably at least 450 mm³, but preferably not more than 750 mm³, and more preferably not more than 700 mm³.

Moreover, in the golf ball of the invention, to optimize the flight trajectory and increase even further the distance, it is preferable for the ball, when struck, to have a coefficient of lift CL at a Reynolds number of 70,000 and a spin rate of 2,000 rpm which is adjusted to at least 700 of the coefficient of lift CL at a Reynolds number of 80,000 and a spin rate of 2,000 rpm, and to have a coefficient of drag CD at a Reynolds number of 180,000 and a spin rate of 2,520 rpm which is adjusted to 0.225 or below.

Multi-piece solid golf balls having the above-described core, intermediate layer, cover and, optionally, an envelope layer can be manufactured by a known process such as injection molding. More specifically, a multi-piece solid golf ball can be obtained by using press molding or injection molding process to fabricate a core composed primarily of a rubber material, using specific injection molds to successively form an envelope layer and an intermediate layer around the core, then injection-molding a cover material over the resulting intermediate layer-encased sphere. Alternatively, another method may be used to form the cover layer in which a pair of half-cups are molded beforehand using the above-described cover material, the intermediate layer-encased sphere is enclosed in these half-cups, and molding under applied pressure is carried out at from 120 to 170° C. for 1 to 5 minutes.

The multi-piece solid golf ball of the invention can be made to conform with the Rules of Golf for use as a game ball, and is preferably set to a ball diameter such that the ball will not pass through a ring having an inside diameter of 42.672 mm and to a weight of generally from 45.0 to 45.93 g.

As described above, the multi-piece solid golf ball of the invention is able to achieve both an increased distance owing to a reduced spin rate, and also an improved durability. The golf ball of the invention provides a good, soft feel on impact, and also has an excellent scuff resistance.

EXAMPLES

Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 5 Comparative Example 1

A solid core having a diameter of 35.2 mm and a weight of 26.9 g was obtained using a core material of the following core formulation and composed primarily of cis-1,4-polybutadiene.

Core Formulation cis-1,4-Polybutadiene 100 parts by weight Zinc oxide 4 parts by weight Barium sulfate 22.39 parts by weight Antioxidant 0.1 part by weight Zinc acrylate 23 parts by weight Dicumyl peroxide 0.6 part by weight 1,1-Bis(tert-butylperoxy)cyclohexane 0.6 part by weight cis-1,4-Polybutadiene: Available as “Br01” from JSR Corporation Zinc oxide: Available from Sakai Chemical Co., Ltd. Barium sulfate: Available as “Precipitated Barium Sulfate 100” from Sakai Chemical Co., Ltd. Antioxidant: Available as “Nocrac NS-6” from Ouchi Shinko Chemical Industry Co., Ltd. Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.

Next, using the respective resin materials shown in Table 1, an envelope layer, an intermediate layer and a cover layer were formed by successive injection-molding over the core. The Dimple Configuration I (302 dimples arranged in the pattern shown in FIG. 2) was used for the dimples on all the balls.

TABLE 1 A B C D E F G H Hytrel 4001 100 Himilan 1605 50 Himilan AM7329 50 50 50 Himilan AM7318 50 50 Himilan AM7315 50 Surlyn 8150 50 Nucrel AN4319 85 100 100 Dynaron 6200P 15 Titanium oxide 2.8 2.0 2.8 2.8 Polyethylene wax 1 1 1 1 Magnesium stearate 100 70 69 1.73 1.73 1.73 1.73 Magnesium oxide 2.8 1.9 1.2 Barium sulfate 20 20 20 Calcium carbonate 1 20 Degree of neutralization (%) 110 100 80 Numbers in the table indicate parts by weight. Hytrel 4001: A polyester elastomer available from DuPont-Toray Co., Ltd. Himilan 1605: An ionomer resin of a copolymer (acid content, 15 wt %), available from DuPont-Mitsui Polychemicals Co., Ltd. Himilan AM7329: An ionomer resin of a copolymer (acid content, 15 wt %), available from DuPont-Mitsui Polychemicals Co., Ltd. Himilan AM7318: An ionomer resin of a copolymer (acid content, 18 wt %), available from DuPont-Mitsui Polychemicals Co., Ltd. Himilan AM7315: An ionomer resin of a copolymer (acid content, 20 wt %), available from DuPont-Mitsui Polychemicals Co., Ltd. Surlyn 8150: An ionomer resin of a copolymer (acid content, 19 wt %), available from E. I. DuPont de Nemours & Co. Nucrel AN4319: A terpolymer available from DuPont-Mitsui Polychemicals Co., Ltd. Nucrel 1560: A copolymer available from DuPont-Mitsui Polychemicals Co., Ltd. Dynaron 6200P: A thermoplastic block copolymer having polyolefin crystalline blocks and a polyethylene/butylene copolymer, available from JSR Corporation. Titanium oxide: Available as “Tipaque R550” from Ishihara Sangyo Kaisha, Ltd. Polyethylene wax: Available as “Sanwax 161P” from Sanyo Chemical Industries, Ltd. Magnesium stearate: Available as “Magnesium Stearate G” from NOF Corporation. Magnesium oxide: Available as “Kyowamag MF150” from Kyowa Chemical Industry Co., Ltd. Barium sulfate: Available as “Precipitated Barium Sulfate 100” from Sakai Chemical Co., Ltd. Calcium carbonate: Available as “Silver W” from Shiraishi Calcium Kaisha, Ltd.

The following ball properties were investigated for the golf balls obtained. Also, flight tests were carried out by the following method, in addition to which the flight performance, feel on impact, scuff resistance and durability to cracking were evaluated. The results are shown in Table 2.

Deflection of Core and Manufactured Ball

The sphere to be tested was placed on a hard plate, and the amount of deformation (mm) by the sphere when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) was measured.

Envelope Layer, Intermediate Layer and Cover Material Hardnesses

The Shore D hardnesses were measured in accordance with ASTM D-2240.

Initial Velocity of Ball

The initial velocity was measured using an initial velocity measuring apparatus of the same type as the USGA drum rotation-type initial velocity instrument approved by the R&A. The spheres to be measured (core, intermediate layer-covered sphere I, and golf ball) were held isothermally at a temperature of 23±1° C. for at least 3 hours, then tested in a room temperature (23±2° C.) chamber. The sphere was hit using a 250-pound (113.4 kg) head (striking mass) at an impact velocity of 143.8 ft/s (43.83 m/s). A dozen balls were each hit four times. The time taken for the balls to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity. This cycle was carried out over a period of about 15 minutes.

Distance with W#1

Each ball was hit ten times at a head speed (HS) of 45 m/s with a TourStage X-Drive 705 (loft angle, 10.5°) driver (manufactured by Bridgestone Sports Co., Ltd.) mounted on a golf swing robot, and the spin rate (rpm) and total distance (m) were measured.

Feel

The feel of the ball when hit by ten top amateur golfers with a driver (W#1) at a head speed (HS) of 40 to 45 m/s was rated according to the following criteria.

-   -   Good: Seven or more of the golfers thought the ball had a good         feel     -   Fair: From four to six of the golfers thought the ball had a         good feel     -   NG: Three or fewer of the golfers thought the ball had a hard         feel

Scuff Resistance

The golf ball was held at a temperature of 23° C. and hit at a head speed of 33 m/s using a pitching wedge mounted on a swing robot machine, following which damage from the impact was visually rated according to the following criteria.

-   -   Exc: No damage whatsoever     -   Good: Damage of a degree that is difficult to visually observe     -   Fair: Damage of a degree that poses no problem whatsoever in         terms of use     -   NG: Damage of a severity, including obliterated dimples, that         precludes further use

Durability to Cracking Under Repeated Impact

The durability of the golf ball was evaluated using an ADC Ball COR Durability Tester produced by Automated Design Corporation (U.S.). The ball was fired pneumatically and made to repeatedly strike two metal plates arranged in parallel. Using the average number of shots required for the ball to crack, the durability was rated according to the criteria indicated below. (Average values were obtained by furnishing four balls of the same type for testing, repeatedly firing each of the four balls until it cracked, and averaging the number of shots required for the respective balls to crack. The type of tester used was a vertical COR durability tester, and the incident velocity of the balls on the metal plates was 43 m/s.)

TABLE 2 Comp. Example Example 1 2 3 4 5 1 Core Diameter (mm) 35.2 35.2 35.2 35.2 35.2 35.2 Deflection (mm) 4.1 4.1 4.1 4.1 4.1 4.1 Envelope Material B A A A A A layer Shore D hardness 45 40 40 40 40 40 Gauge (mm) 1.2 1.2 1.2 1.2 1.2 1.2 Intermediate Material C C C C C D layer Shore D hardness 49 49 49 49 49 50 Gauge (mm) 1.2 1.2 1.2 1.2 1.2 1.2 Degree of 100 100 100 100 100 80 neutralization Cover Material E E F G H E Shore D hardness 65 65 70 65 63 65 Gauge (mm) 1.35 1.35 1.35 1.35 1.35 1.35 Manufactured Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 ball Deflection (mm) 2.9 3.0 2.9 3.0 3.1 3.0 Distance Carry (m) 215 214 216 214 213 213 (HS, 45 m/s) Total distance (m) 233 232 234 232 230 229 Ball Spin rate (rpm) 2,600 2,600 2,550 2,620 2,660 2,680 performance Feel fair good good good good good Scuff resistance good good Exc good fair good Durability 100 100 80 95 120 100 Initial velocity (m/s) 77.4 77.2 77.4 77.2 77.1 77.1

From the results in Table 2 above, the golf balls of Examples 1 to 5 according to the present invention all had both an excellent distance and an excellent durability, in addition to which they had a good feel and an acceptable scuff resistance. By contrast, in Comparative Example 1, the durability was excellent, but the ball had an excessive spin receptivity, as a result of which a good distance was not achieved on shots with a W#1. 

1. A multi-piece solid golf ball comprising a core, an intermediate layer encasing the core, and a cover layer which encases the intermediate layer and has a plurality of dimples on an outer surface thereof, wherein the intermediate layer is formed primarily of a material which is comprised of: 100 parts by weight of a resin component composed of, in admixture, a base resin of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplastic elastomer in a weight ratio between 100:0 and 50:50; (c) from 5 to 150 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and (d) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in the base resin and component (c), and in which at least 82 mol % of acid groups in the base resin and component (c) are neutralized; and wherein the cover layer is formed of a thermoplastic resin composed primarily of an ionomer resin and has a Shore D hardness of from 50 to
 85. 2. The multi-piece solid golf ball of claim 1, wherein the ionomer resin in the cover layer includes at least 16 wt % of an unsaturated carboxylic acid, and the cover layer includes from 5 to 35 parts by weight of a granular inorganic filler per 100 parts by weight of the thermoplastic resin.
 3. The multi-piece solid golf ball of claim 1, which further comprises, between the core and the intermediate layer, an envelope layer composed primarily of a polyester elastomer.
 4. The multi-piece solid golf ball of claim 1, wherein the total number of dimples is from 250 to
 342. 